Alexander Selkirk (1676 - 1721) was a Scottish sailor who became marooned on an uninhabited island for more than four years. He is thought to be the real-life inspiration for Daniel Defoe's novel Robinson Crusoe. In 1829 an account of his experience - The Life and Adventures of Alexander Selkirk - was written by John Howell. He was also immortalised in William Cowper's poem The Solitude Of Alexander Selkirk.
On page 7 of Possession, Byatt lists 'The Solitary Thoughts of Alexander Selkirk' as one of Randolph Ash's poems.